Bacillus thuringiensis is a Gram-positive spore forming soil bacterium characterized by its ability to produce crystalline inclusions that are specifically toxic to certain orders and species of insects, but are harmless to plants and other non-targeted organisms. For this reason, compositions including Bacillus thuringiensis strains or their insecticidal proteins can be used as environmentally-acceptable insecticides to control agricultural insect pests or insect vectors for a variety of human or animal diseases.
Crystal (Cry) proteins (delta-endotoxins) from Bacillus thuringiensis have potent insecticidal activity against predominantly Lepidopteran, Dipteran, and Coleopteran larvae. These proteins also have shown activity against Hymenoptera, Homoptera, Phthiraptera, Mallophaga, and Acari pest orders, as well as other invertebrate orders such as Nemathelminthes, Platyhelminthes, and Sarcomastigorphora (Feitelson (1993) The Bacillus Thuringiensis family tree. In Advanced Engineered Pesticides, Marcel Dekker, Inc., New York, N.Y.) These proteins were originally classified as CryI to CryV based primarily on their insecticidal activity. The major classes were Lepidoptera-specific (I), Lepidoptera- and Diptera-specific (II), Coleoptera-specific (III), Diptera-specific (IV), and nematode-specific (V) and (VI). The proteins were further classified into subfamilies; more highly related proteins within each family were assigned divisional letters such as Cry1A, Cry1B, Cry1C, etc. Even more closely related proteins within each division were given names such as Cry1C1, Cry1C2, etc.
A new nomenclature was recently described for the Cry genes based upon amino acid sequence homology rather than insect target specificity (Crickmore et al. (1998) Microbiol. Mol. Biol. Rev. 62:807-813). In the new classification, each toxin is assigned a unique name incorporating a primary rank (an Arabic number), a secondary rank (an uppercase letter), a tertiary rank (a lowercase letter), and a quaternary rank (another Arabic number). In the new classification, Roman numerals have been exchanged for Arabic numerals in the primary rank. Proteins with less than 45% sequence identity have different primary ranks, and the criteria for secondary and tertiary ranks are 78% and 95%, respectively.
The crystal protein does not exhibit insecticidal activity until it has been ingested and solubilized in the insect midgut. The ingested protoxin is hydrolyzed by proteases in the insect digestive tract to an active toxic molecule. (Höfte and Whiteley (1989) Microbiol. Rev. 53:242-255). This toxin binds to apical brush border receptors in the midgut of the target larvae and inserts into the apical membrane creating ion channels or pores, resulting in larval death.
Delta-endotoxins generally have five conserved sequence domains, and three conserved structural domains (see, for example, de Maagd et al. (2001) Trends Genetics 17:193-199). The first conserved structural domain consists of seven alpha helices and is involved in membrane insertion and pore formation. Domain II consists of three beta-sheets arranged in a “Greek key” configuration, and domain III consists of two antiparallel beta-sheets in “jelly-roll” formation (de Maagd et al., 2001, supra). Domains II and III are involved in receptor recognition and binding, and are therefore considered determinants of toxin specificity.
Aside from delta-endotoxins, there are several other known classes of pesticidal protein toxins. The VIP1/VIP2 toxins (see, for example, U.S. Pat. No. 5,770,696) are binary pesticidal toxins that exhibit strong activity on insects by a mechanism believed to involve receptor-mediated endocytosis followed by cellular toxification, similar to the mode of action of other binary (“A/B”) toxins. A/B toxins such as VIP, C2, CDT, CST, or the B. anthracis edema and lethal toxins initially interact with target cells via a specific, receptor-mediated binding of “B” components as monomers. These monomers then form homoheptamers. The “B” heptamer-receptor complex then acts as a docking platform that subsequently binds and allows the translocation of an enzymatic “A” component(s) into the cytosol via receptor-mediated endocytosis. Once inside the cell's cytosol, “A” components inhibit normal cell function by, for example, ADP-ribosylation of G-actin, or increasing intracellular levels of cyclic AMP (cAMP). See Barth et al. (2004) Microbiol Mol Biol Rev 68:373-402.
The intensive use of B. thuringiensis-based insecticides has already given rise to resistance in field populations of the diamondback moth, Plutella xylostella (Ferré and Van Rie (2002) Annu. Rev. Entomol. 47:501-533). The most common mechanism of resistance is the reduction of binding of the toxin to its specific midgut receptor(s). This may also confer cross-resistance to other toxins that share the same receptor (Ferré and Van Rie (2002)).